1. Field of the Invention
The present invention generally relates to a method of forming a resist pattern, and more particularly to an improved method of forming a resist pattern permitting control of a resist profile of a chemically amplified-type resist. The present invention also relates to a method of forming semiconductor device using the resist pattern.
2. Description of the Background Art
In the field of photolithography, a chemically amplified-type resist is currently proposed as a resist having high sensitivity as well as high resolution. This resist has a feature that it is baked after exposure to light, and this post exposure bake (hereinafter referred to as "PEB") accelerates pattern forming reaction with an acid generated during the exposure as a catalyst, and thus pattern imaging is completed.
FIGS. 15-18 are cross sectional views of a semiconductor device at successive steps of a conventional method of forming a fine resist pattern using a chemically amplified-type resist.
Referring to FIG. 15, a substrate 1 is coated with a chemically amplified-type resist 2. Note that the surface of substrate 1 has been processed with HMDS (hexamethyldisilane) or the like before coating with the resist, to ensure close contact between substrate 1 and resist 2.
Resist 2 is formed of poly (p-t-butyl oxy-carbonyl oxy-styrene) with t-butyl oxy-carbonyl (hereinafter referred to as "t-BOC") coupled to poly-phydroxy styrene, as shown, for example, in FIG. 19A. This composition is only by way of example, and other compositions may also be used. In FIG. 19A, n is a natural number representing a degree of polymerization. Resist 12 includes an acid generating agent, i.e., triphenyl sulfonium trifluoro methane sulfonate, as shown in FIG. 19B.
Once substrate 1 has been coated with resist 2, it is pre-baked to remove a solvent or the like included in resist 2.
An acid film 3 is formed on resist 2, which is made, for example, of water-soluble polymer such as polyvinylpyrrolidone or polyacrylic acid. Acid film 3 selves for preventing intra-film multiple reflections. Another function expected of the film here will be described later. Thereafter, baking is conducted again to remove a solvent in acid film 3.
With reference to FIG. 16, resist 2 is selectively exposed to light using a mask 4, to form an exposed portion 5 and an unexposed portion 6.
At the exposed portion, the acid generating agent is irradiated with light, and thus generates a protonic acid, as shown in FIG. 20.
Referring to FIG. 17, resist 2 is subjected to PEB. At this time, the protonic acid causes t-BOC groups to leave from poly (p-t-butyl oxy-carbonyl oxy-styrene), so that hydroxyl groups (phenol groups) are generated. Thus generated poly-p-hydroxy styrene is soluble in alkali.
Next, how the t-BOC groups leave from the base resin will be described in more detail with reference to FIG. 22.
With reference to FIG. 22(a), resist 2 is irradiated with light. Accordingly, the acid generating agent in the resist decomposes to generate a protonic acid (H.sup.+) (see FIG. 22(b)). As shown in FIG. 22(c), subsequent baking of resist 2 causes the t-BOC groups to leave from poly (p-t-butyl oxy-carbonyl oxy-styrene), thereby generating hydroxyl groups. As the baking is continued, more t-BOC groups are generated (FIG. 22 (d)). Ultimately, as seen in FIG. 22(e), the base resin is converted to polyvinylphenol, that is soluble in an alkaline developer solution.
Generally, a chemically amplified-type resist is susceptible to acid and base existing within the resist or entering from the outside. An acid film 3 is thus formed on the resist 2 as a protective film. Since this protective film is an acid film, however, acid is fed from acid film 3 to the surface of resist 2 at the unexposed portion 6. The reference numeral 7 in FIG. 17 shows a portion which has been supplied with the acid and thus become soluble in alkali.
With reference to FIG. 18, when developed with an alkaline solution with 2.38% tetramethylammonium hydroxide (TMAH) 2, the alkali-soluble portion is removed to form a fine resist pattern. Using this resist pattern, a bit line or the like is patterned.
As described above, in a conventional method of forming a fine resist pattern using a chemically amplified-type resist, acid is supplied from acid film 3 to resist 2 even at the unexposed portion, as shown in FIGS. 17 and 18. Accordingly, resist pattern 8 obtained after the development has a rounded top portion. That is, a rectangular resist profile can not be attained as desired.
If acid film 3 is not formed on resist 2, amine (ammonia etc.) will penetrate into resist 2 from the outside, and as shown in FIG. 23, the top portion of resist pattern 8 will expand in the horizontal direction, resulting in a so-called T-shaped resist pattern. Either in this case, a desirable, rectangular resist cannot be attained.